The world is facing fluctuations in crude oil prices as well as challenges to energy security, economic stability and growth. Further environmental concerns related to climate change due to the ‘greenhouse effect’ is coming more and more in focus. Furthermore a number of conventional energy sources such as oil are being depleted. This calls for a more efficient and sustainable use of resources, including non-conventional and alternative resources.
Hence, there is a large and increasing global interest in new technologies for the production of liquid hydrocarbons from low value abundant resources such as lignite, peat, biomass, residues and waste. A general characteristic of such low value resources is that they typically have high moisture content, an oxygen content on a dry ash free basis in the range 20-60%, and an ash content ranging from a few percent to more than 50% by weight, which results in a low heating value as received.
Technologies for production of nonconventional liquid hydrocarbons are known e.g. production of liquid hydrocarbons from coal has been known for more than 150 years. Pyrolysis or high temperature carbonization is another well known route for production of liquid hydrocarbons from solid fuel. Depending on the specific process the input stream may be heated to a temperature in the range 450 to 1000° C. in the absence of oxygen, driving of the volatile compounds and leaving a coke product. The hydrocarbon yield can be wide varying and ranges from 10 to 75% depending on the volatile content of the specific input streams and process conditions. In general fast heating (fast pyrolysis) and short residence time provides the highest yields. However, pyrolysis is limited to dry input streams e.g. moisture contents up to approximately 10% by weight. Further as only very limited conversion of the liquid hydrocarbon produced occurs during processing, the liquid hydrocarbons produced have a high oxygen and water content, and the liquid hydrocarbons produced consequently have a low heating value. Further, the liquid hydrocarbons are not mixable with petrodiesel and petrocrude, and are corrosive and susceptible to polymerization which makes long term storage difficult. This limits the direct use of such pyrolytic hydrocarbon liquids. Upgrading of pyrolytic hydrocarbons may be performed by hydrodeoxygenation or by addition of hydrogen during the pyrolysis process. However, though such hydrogenation processes are technically feasible, they will add significantly to the production costs as no oxygen is removed by the pyrolysis, and production of hydrogen is relatively expensive.
Indirect liquefaction of coal by first producing a syngas by thermal gasification and subsequent conversion into liquid hydrocarbons by the Fischer-Tropsch route has been practiced by Sasol in South Africa since the 1950's. Shell and ExxonMobil have developed similar technologies for production of liquid hydrocarbons from natural gas. Indirect gasification is characterized by being very capital intensive and having relatively low efficiencies. Typically the energy efficiency for conversion from coal to liquid hydrocarbons is in the range 30-50%.
Production of liquid hydrocarbons by dissolution of coal in a solvent in the presence of high hydrogen pressures and iron catalysts to produce high boiling liquids is known as the Bergius, Pott Broche or I. G. Farben process and was used to produce gasoline during the Second World War. Common features are dissolution of a high proportion of coal in a solvent at elevated temperature, followed by hydro-cracking of the dissolved coal with hydrogen and a catalyst. The processes differ in the number of stages, process conditions and specific catalysts applied.
The production of liquid hydrocarbons from feedstock other than coal is also being conducted by the pyrolysis, indirect, and direct liquefaction techniques described above. However, common for all of them is that they all require relatively dry input streams. A fundamental issue is the difference in the stoichiometry of the input stream and liquid hydrocarbon fuels.
This fundamental issue results in an indispensable need for hydrogen addition and/or removal of carbon during the processing for adaptation of the H/C ratio and removal of oxygen. Removal of carbon as char and CO2 reduces the maximum obtainable yields of the desired hydrocarbons, whereas production of hydrogen is relatively expensive and adds significantly to the complexity and reduces the efficiency of such processes. Hence to be viable such processes require a very large scale and thereby become very capital intensive (UK DTI, Coal Liquefaction, Cleaner Coal Programme, Technology Status Report 010, October 1999).
Hence, there is a large interest in developing improved production techniques for liquid hydrocarbons not suffering from the drawbacks described above. Conversion of the feedstock in pressurized water at elevated temperatures is a route which has attracted significant attention over recent decades. Such techniques are generally called hydrothermal processing, and generally convert the feedstock into liquid hydrocarbon product, a char product, a water phase comprising water soluble organics, a gas and a mineral product.
An advantage of hydrothermal processing is that water is kept under pressure so that it is maintained in its liquid and/or supercritical state which means that no phase transition into steam occurs during processing. Hence, the energy loss, in the form of latent heat of evaporation, need not be supplied, and thus energy consuming processes such as evaporation or distillation are eliminated. This renders such processes very energy efficient particularly for wet input streams.
Water, in the vicinity of its critical point (374° C., 221 bar) obtains physical properties which are very different from water at ambient conditions e.g. the dissociation product of water is more than three orders of magnitude higher, it changes its polarity from a polar solvent to a non-polar solvent, interphase mass and heat transfer resistances are significantly reduced and mass- and heat transfer rates are therefore enhanced.
Due to these properties of water in the vicinity of its critical point, water may serve both as a reaction medium, a catalyst for acid and base catalyzed reactions and as a reactant and source of hydrogen in the conversion process.
Hence hydrothermal processing holds the potential to reduce the oxygen content of wet oxygenated feedstock with lower parasitic energy losses and with less hydrogen required due to formation of hydrogen in situ.
An excellent review of the state of the art of such hydrothermal processes and characteristic chemical reactions for conversion of organic macromolecules is given in A. Peterson et al, “Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies, Energy Environ. Sci., 2008, 1, 32-65.
Deoxygenation goes through dehydration, decarboxylation and hydrogenation reactions. However, the reaction pathways are complex and are to a large extent unknown except for simple molecules. Carbonaceous macromolecules may undergo various reactions including hydrolysis, dehydration, decarboxylation, steam reforming, water gas shift, steam cracking, Bouduard reaction, hydrogenation, methanation, Fischer-Tropsch, aldol condensation, esterification, methanol synthesis etc. The rate of the individual reactions and the extent to which conversion proceeds via specific reaction pathways depends on a number of factors.
Processes differ in the specific operating conditions and process design and layout being applied e.g. the feedstock, the dry solid content in the feed, the ash content of the feed, the operating pressure and temperature, the pH, the catalysts and other additives present in different parts of the process, the residence time in the different parts of the process, the heat integration, the separation techniques applied including further product handling and upgrading etc.
These factors all influence the distribution, yields and quality of the products produced i.e. the amount and quality of liquid hydrocarbons, the amount and quality of char, the amount of organics contained in the water phase, and the amount and quality of gas, and the amount and quality of mineral product. Further they influence the overall efficiency of the process i.e. the parasitic energy loss and overall energy recovery in desired product(s), amount of consumables used, the robustness and complexity of the process as well as the overall process economics.
Several hydrothermal conversion processes of biomass and other carbonaceous macromolecules are in the development or demonstration including hydrothermal processes producing char or a solid residue as the main product, thermal wet gasification, catalytic gasification and hydrothermal liquefaction to produce liquid hydrocarbons.
Processes for production of coke/char product by supercritical hydrothermal dewatering and/or partly depolymerization have been developed. Examples of hydrothermal processes being commercialized are the Slurycarb process by Enertech (N. L. Dickinson, WO95/014850, www.enertech.com), the K-fuel process by Evergreen Energy (R. F. Hogsett, EP2,287,279, www.evergreen.com), and the JGC Coal Fuel process by JGC Corporation (M. Tsurui et al, U.S. Pat. No. 6,132,478, www.jgc.co.jp/enindex.html). Common to these processes are is the aim to produce a partly depolymerized char product as the main product and that they operate at relatively low pressure (50-150 Bar) and temperature (200-300° C.).
Thermal wet gasification aims at producing gas by thermal decomposition without applying a heterogeneous catalyst. Typically such processes operate at temperatures in the range 500-700° C., and pressures above the critical pressure of water. Corrosion is severe at these conditions, and places very high demands on the materials of construction (A. Peterson et al, 2008). Hence, a considerable interest is directed to gasification processes applying a heterogeneous catalyst to decrease the temperature required for said gasification to proceed with reasonable rate and yield (A. Peterson et al, 2008; M. Osada et al, 2006; F. Vogel et al, US2009/0126274; D. C. Elliott et al, WO2009/099684). Catalytic gasification may proceed at operating temperatures in the range 400 to 500° C. However, the use of heterogeneous catalysts requires efficient removal of suspended particles prior to contact with said heterogeneous catalyst to avoid clogging of the reactor (A. Peterson et al, 2008; F. Vogel et al; US2009/0126274; D. C. Elliott et al, WO2009/099684). Progress is being made in this direction (F. Vogel et al; US2009/0126274; D. C. Elliott et al, WO2009/099684) No hydrothermal gasification plant has yet been commercialized (A. Peterson et al, 2008).
Hydrothermal processes for production of liquid hydrocarbons from carbonaceous materials are generally performed at a pressure sufficient to avoid vaporization of the fluid, and at lower temperatures than hydrothermal gasification processes, to maximize yield of liquid hydrocarbon products. Typically the pressure is in the range 40 to 200 bar and the temperature in the range 200 to 370° C. (A. Peterson, 2008). Some of the most significant prior processes are described below.
Shell developed the so-called HTU process for production hydrocarbon containing liquids from biomass (Annee et al, EP 0,204,354). The process converts biomass products such as wood at temperatures in the range 300 to 380° C. and a pressure above the boiling point of water, preferably in the range 150 to 250 bar and residence times from 3 to 10 minutes. No catalyst was used in the process. Heating was performed by a combination of indirect heating and heating by direct steam injection. An oil yield of 30-50% calculated as the ratio of the mass of oil to the mass of dry biomass feed was obtained from wood chips as well as char (carbon) in an amount of 10 to 22% by weight, 20-25% gas by weight and 20-23% water and water-solubles by weight. The oil produced contained up to 20% oxygen by weight. An embodiment comprises recycling a substantially aqueous liquid to a pretreatment step to increase the thermal efficiency and reduce water consumption.
A further development of the above HTU process is disclosed by Van de Beld et al in U.S. Pat. No. 7,262,331. The further development include pressurizing the feedstock to preferably 130 to 180 bar, heating to a temperature in the range 180-280° C. and maintaining it at these conditions in a period up to 60 minutes to produce a reaction mixture, which is further heated to a temperature in the range 280 to 350° C. over a period of up 60 minutes. An option includes separation of a liquid fraction containing fermentable compounds from the mixture prior to heating to the reaction temperature. Heating is performed by a combination of indirect heating, direct injection of steam, direct injection of a preheated CO2 containing gas and/or an oxygen containing gas. The process results in a liquid hydrocarbon crude with an oxygen content of 10-25% by weight, a mineral fraction 0.5-10% by weight, and with about 50% of the liquid hydrocarbons boiling above 450° C. The heavy fraction has an oxygen content of 10-20% by weight and mineral content of 0.5 to 25% by weight, and the light fraction has an oxygen content of 5 to 25% and a mineral content of less than 0.5% by weight.
Yokoyama et al (U.S. Pat. No. 4,935,567) discloses a process for producing a liquid hydrocarbon product from cellulotic biomass such as wood by treatment of the biomass by conversion of the biomass at a pressure of 3 to 100 atm and a temperature from 250 C to 400° C. (372 to 378° C. preferred) in the presence of a neutral oxygen-containing organic liquid in the form of alcohols, ketones, ethers, esters and mixtures thereof. A particularly preferred embodiment is when said neutral oxygen-containing organic liquid is acetone. The oxygen containing liquid is claimed to accelerate the reactions and makes it easy to separate the liquefied product from the reaction mixture. Another embodiment include the use of an alkaline catalyst in a concentration of 1 to 10% by weight of the dry biomass. The alkaline catalyst may be used in an amount so that the reaction mixture has a pH in the range 10-14 and preferably in the range. The dry solid content of the biomass is preferably in the range 5 to 20% by weight (5-20 parts). The product was separated by decanting (oil phase heavier than water), and subsequent distillation to distill off water. The liquefied hydrogen products produced had calorific values between 24.5 MJ/Kg and 35.5 MJ/kg and contained 14-31% oxygen by weight. Most of the oils solidified at room temperature and were not considered to be stable at room temperature. An experiment conducted at 375° C. produced oil that didn't solidify at room temperature. Though the patent discloses some parts which may be attractive, the yields achieved are considered as very low i.e. 20-25% of the dry biomass weight. The oxygen content of the produced liquid hydrocarbon product is considered to be high despite the relatively high calorific values. Further be noticed that the pressure being applied is not high enough to ensure that the fluid mixture is in a single phase. Assuming that the fluid mixture comprises pure water, the fluid will be on a vapor phase in the whole temperature range from 200 to 400° C., and at 100 atm the fluid will be on a liquid form up to 312° C., and on a vapor form from 312 to 400° C. This is considered insufficient according to the present invention.
Humfreys (WO2009/015409) discloses a process for converting organic matter such as lignite or brown coal, lignin, cellulose, hemicellulose, organic waste, plastic or a generic polymer into products including mixing it with a supercritical liquid comprising one or more of the group consisting of water, methanol, and ethanol at a pressure greater than 220 bar (up to more than 300 bar) and temperatures in the range 350 to 420° C. The products produced by the process include heavy oil petroleum fractions referred to as oil, asphaltenes and pre-asphaltenes, and also yielding residual char, gas (mostly carbon dioxide) and produced water as the main products. The process disclosed is in many ways very similar to the HTU process described above in relation to the disclosures by Annee et al and Van de Beld et al with major differences being the presence of methanol and/or ethanol in the fluid and/or operation at higher pressures and/or temperatures.
Iversen et al (WO2006/1170002A3) discloses a catalytic process, wherein organic material is converted into converted into hydrocarbon fuels with high efficiency. In this process, organic matter such as biomass, waste and sludges is converted by pressurizing said organic matter to a pressure of at least 225 bar, and heating said fluid comprising said organic matter to a temperature of at least 200° C. in the presence of a homogeneous catalyst (comprising at least one compound of an element of group IA of the periodic table of elements, such as at least one compound of potassium and/or sodium), and subsequently contacting the fluid containing organic material with a heterogeneous catalyst (comprising a compound of at least one element of group IVB of the periodic table such as zirconia and/or titania and/or alpha alumina at a temperature of up to 374° C., while maintaining the fluid at a pH of least 7. In a preferred embodiment described, the heating is performed in a sequential manner, and the hot effluent from the heterogeneous reactor, containing reaction products and/or intermediate reaction products, is at least partly recycled and mixed with the feed mixture after heating to more than 200° C. The combined fluid of the incoming feed mixture and re-circulated reactor effluent is further heated to reactor temperature in a trimheater. Accompanying examples indicate up to 40% of the carbon and up to 76% of the energy contained in the feed being recovered as a liquid hydrocarbon (oil).
Despite that hydrothermal technologies have many potential benefits over conventional methods of processing biomass and other organic macromolecules to useful fuels and chemicals, the fact remains that these technologies have yet not been widely commercialized (A. Peterson et al, 2008).
There are a number of challenges that may be addressed to improve the effectiveness of processing. These include:                Gasification processes operating without a heterogeneous catalyst at temperatures in the in the range 450-700° C., demand specialized materials to withstand the high temperature and corrosive environment at these conditions (e.g. A. Peterson et al, 2008).        Effective and economically viable processes demand a feedstock at high dry solid loading e.g. at least 20% by weight. Size reducing and feeding of such feedstock is difficult as it may have a solid appearance and high viscosity, particularly for fibrous materials, and may block orifices and contra valves in pumps. Inadequate pretreatment and/or homogenization and/or pump design has limited a number of processes to operate at low dry solids content, which challenges the economy of such processes (e.g. A. Peterson et al, 2008; M. Osada et al, 2006).        Some feedstock contains high amount of salts and inorganics that can lead to precipitation, fouling and plugging of pipes, heat transfer surfaces, reactors and other process equipment if not properly managed (e.g. A. Peterson et al, 2008; Osada, et al, 2006).        Processes applying heterogeneous catalysts for production of syngas or syncrudes are applied in a number of processes to lower the operating temperature and/or increase the yield of desired product. The success of these processes has been varying. A number of processes have been developed wherein inadequate catalysts which do not withstand hydrothermal processing conditions have been applied. Further the application of such heterogeneous catalysts are prone to clogging of reactors and catalysts pores if not properly designed for high loads of impurities and/or efficient removal of suspended particles prior to said catalytic reactors is performed (A. Peterson et al, 2008, Vogel et al, US2009/0126274A1, Elliott et al, WO2009/099684A3).        Processes are susceptible to formation of tar and chars if process steps and operating conditions are not selected properly. The formation of tars and char may result in increased fouling and result in a less efficient process due to formation of solid residues instead of desired products (Vogel et al, US US2009/0126274A1).        Some feedstocks such as lignite, sub-bituminous coals and high-lignin containing biomasses are susceptible to tar and char formation, and often produce significant amount of solid residues.        Water soluble organic compounds in prior art hydrothermal processes for liquid hydrocarbon production can comprise 5 to 70% of the carbon and to 60% of the energy contained in said carbonaceous material being fed to the process, depending of the specific carbonaceous material and/or combination of carbonaceous materials being converted, specific process steps and process parameters for said hydrothermal conversion process (e.g. Hammerschmidt et al, 2011). Besides representing a process loss reducing the yield of desired products, such water soluble organic products may be considered as pollutants that increases the treatment and purification requirements of the water effluent.        Homogeneous catalysts such as potassium and sodium are well known to enhance the degradation and conversion of organic macromolecules in the feed mixture and suppress formation of coke and char for both gasification and liquefaction processes (A. Peterson, 2008; S. Karagöz et al, 2006; T. Bhaskar et al, 2006; Hammerschimidt, 2011). However, such homogeneous catalysts are relatively expensive, and must be recovered or reused in order to achieve an economically viable process (A. Peterson et al; 2008).        
Hence an improved process and apparatus for production of liquid hydrocarbons as the main product and not suffering from the problems and disadvantages from the prior art is advantageous and desirable.
Accordingly, it is an object of the invention to provide an improved process for the production of liquid hydrocarbons, and further to provide an improved apparatus for the production of liquid hydrocarbons.